Simulating supernovae on supercomputers

Tony Mezzacappa sits before a computer visualization of an exploding star. The visualization illustrates convection in a core-collapse supernova explosion. The red plumes, composed of material that has been heated by neutrinos, are rising, and the yellow fingers, composed of cooler material, are moving inward. (Photo by Curtis Boles)Click here for more photos.

Billions of years ago, the universe became the stage
for spectacular stellar explosions. These momentous
celestial events marked the end of a star’s life but
planted the seeds that would ultimately result in the
explosion of life on the earth. After millions of years
of evolution, massive stars were disrupted in a
creative process. These core-collapse supernovae
spewed forth lightweight, life-giving elements such
as carbon and oxygen that eventually reached the
regions out of which our solar system formed. They
also synthesized heavy elements and disseminated
them to interstellar space. In addition to the elements
they created, supernovae also left their mark in the
form of neutron stars and black holes.

“Life as we know it would not exist if not for these
incredible explosions of stars,” says Tony
Mezzacappa, task leader for astrophysics theory in
ORNL’s Physics Division. “When stars die in these
explosions that generate energy at the rate of billions
upon billions of watts, elements necessary for life
are strewn throughout our galaxy and become part of
the ‘soup’ from which our solar system formed.”

A major challenge for computational astrophysicists is to solve the “supernova problem,”
using massively parallel supercomputers to model stars greater than 10 times the mass of
the sun. One goal is to predict whether these stars will explode like Supernova 1987A (a
much observed supernova in a nearby galaxy). The other is to predict all of the observed
phenomena associated with such stellar explosions. Three-dimensional (3D) simulations
that can be run only on terascale supercomputers (such as the IBM supercomput-ers at
ORNL) are being developed to do just that.

A core-collapse supernova explosion occurs in
only a few hours in a star that has evolved over
millions of years. This event is thought to be
caused by a shock wave that arises when the
star’s iron core collapses on itself, compressing
its subatomic particles to the point where they
repel each other and force the core to rebound.
Astrophysicists believe the shock wave stalls
while trying to propagate through the stellar
core and is reenergized by neutrino heating.
Neutrinos are particles with no charge and
small mass that interact very weakly with
matter. At the core of a supernova is a neutrino
“bulb” that radiates heat and energy at the
staggering rate of 1045 watts.

ORNL leads the field in simulations of neutrino
transport and must now apply this expertise to developing 3D simulations that will
explore the role played by convection—transfer of heat by the circulation of the core’s
proton-neutron fluid—in aiding this shock revival process, as well as the role played by
the star’s rotation and magnetic field. ORNL researchers are also interested in using these
simulations to predict the emitted gamma rays and gravitational waves (ripples in
space-time) from supernovae.

Supported by $9 million in funding over five years from the
Department of Energy’s Office of Science initiative called Scientific
Discovery through Advanced Computing (SciDAC), ORNL, the
National Center for Supercomputer Applications (NCSA), and eight
universities plan to obtain a detailed understanding of how a star
explodes. Their approach is to perform 3D simulations of the
radiation of the enormous amounts of neutrino energy and the
resulting turbulent fluid flow (hydrodynamics) that together may
propel material into outer space. Computational predictions will be
made consistent with data obtained from astronomical observations.

“Thanks to the growing wealth of observational data from ground-
and space-based facilities and the growing computing power
afforded by massively parallel supercomputers at ORNL and
elsewhere, we are presented with a unique opportunity to finally
solve one of nature’s most important mysteries,” says Mezzacappa.

The simulations will uncover how supernovae synthesize elements
and disseminate them into interstellar space for processing by other
astrophysical systems. Additionally, they hope to determine how the
“neutrino-driven wind,” which arises from the proto-neutron star
left behind after the explosion, synthesizes elements heavier than
iron in a process of rapid neutron capture.

The simulations will also be important in ultimately understanding
how the cooled-down remnants of supernovae give rise to neutron
stars and black holes, creating the basic building blocks of rotating
neutron star (pulsar) and X-ray binary systems.

“We will try to predict whether a black hole or a neutron star will
be left behind in the next supernova explosion in our galaxy,” says
Mezzacappa. “These events occur in our galaxy two or three times
each century.”

One of the collaborating institutions is the University of Tennessee
(UT), where Jack Dongarra and research faculty member Victor
Eijkhout will be working on mathematical solutions (algorithms) to
help solve the equations that govern the motion of neutrinos through
the stellar material.

In addition to UT, ORNL’s collaborators for the project are the State University of New
York at Stony Brook, the University of Illinois at Urbana-Champaign, the University of
California at San Diego, the University of Washington, Florida Atlantic University, North
Carolina State University, and Clemson University. Mezzacappa’s co-investigators at
ORNL are David Dean and Michael Strayer, both of the Physics Division, and Ross
Toedte of the Computer Science and Mathematics Division (CSMD).

The ORNL-centered SciDAC team led by Mezzacappa is also working with five other
SciDAC teams (“ISICs”) on issues that include scalable solution of large sparse linear
systems of equations, code architecture and design, management and analysis of terascale
datasets, code performance, and adaptive meshes, as well as with a supporting “base
project” to address networking issues for this distributed collaboration. These
collaborations involve a number of ORNL staff in CSMD.

This “mother of all applications,”
as Mezzacappa calls the
core-collapse supernova problem,
should provide new insights into
radiation transport and fluid flow
relevant to many phenomena. “Our
work addresses very broad
themes important to DOE’s
national mission,” Mezzacappa
says. “Our ability to model the movement of radiation through matter may help ad-vance
DOE’s energy and basic research missions.”

Examples of interest to DOE are combustion processes in internal combustion engines,
effects of increased atmospheric greenhouse gas concentrations on future climate,
simulated nuclear weapon explosions (stockpile stewardship), production of fusion
energy reactions in hot plasmas, and the effects of radiation therapy on tumors and normal
tissue. Supernova simulations on supercomputers will likely be a shining star in the
astrophysics and other scientific communities.

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